Optical recording media typically comprise an optical recording layer provided on a substrate. For media such as magneto optic recording media and WORM (write-once-read-many) optical recording media, the optical recording layer generally contains a thin film rare earth transition metal alloy such as gadolinium-cobalt (Gd-Co), gadolinium-iron (Gd-Fe), terbium-iron (Tb-Fe), dysprosium-iron (Dy-Fe), Gd-Tb-Fe, Tb-Dy-Fe, Tb-Fe-Co, terbium-iron-chromium (Tb-Fe-Cr), gadolinium-iron-bismuth (Gd-Fe-Bi), Gd-Co-Bi, gadolinium-iron-tin (Gd-Fe-Sn), Gd-Fe-Co, Gd-Co-Bi, and Gd-Dy-Fe. Such alloys are described, for example, in U.S. Pat. No. 4,822,675. For media such as compact disks, the optical recording layer may be a layer of reflective material, for example an aluminum or aluminum alloy, having a patterned, information-bearing surface.
Many of the materials which are suitable for the optical recording layer of optical disks react strongly with oxygen and other elements which may be present in the environment in which optical recording media are used. Furthermore, the substrate itself may contain impurities which react with the optical recording layer. Thus, transparent dielectric films may be deposited on one or both sides of the optical recording layer to protect it. Such dielectric films are described, for example, in U.S. Pat. Nos. 4,833,043 and 4,917,970.
Optionally a reflective layer may be incorporated into optical recording media so that incident light that passes through the optical recording layer a first time is reflected and passes back through the optical recording layer a second time. Such reflection increases the magneto optic rotation of incident light because the so-called Faraday effect is added to the so-called Kerr effect.
The reflective layer may be incorporated into a magnetic recording medium such that the optical recording layer is interposed between the substrate and the reflective layer. For such media, transparent substrates are used so that incident light passes first through the substrate, then passes through the optical recording layer, and then is reflected by the reflective layer back through the optical recording layer. Such media are known as substrate incident media. Alternatively, when the optional reflective layer is disposed between the substrate and the optical recording layer, the read and write beams will not be directed through the substrate. Such a medium is known as an air incident medium, although generally there is at least one layer between the optical recording layer and the air.
For substrate incident media, the substrate is typically formed from polycarbonate. Polycarbonate substrates have excellent rigidity, dimensional stability, transparency, and impact strength, but unfortunately have poor abrasion resistance. Consequently, polycarbonate substrates are susceptible to physical damage from scratches, abrading, and the like.
To protect the substrate from physical damage, a "hard coat" layer is coated onto the substrate to form a protective barrier between the substrate and the air. For example, Japanese Kokai No. JP02-260145 describes a hard coat layer that is coated onto the substrate of an optical card. The hard coat layer is formed from an electron-beam or ultraviolet radiation curable resin. The hard coat layer of Japanese Kokai No. JP02-260145 also includes a surface slipping agent, i.e., a lubricant.
Static charge build-up attracts dust to the hard coat layer of optical recording media, which can prevent read and write beams from reaching the optical recording layer during writing or reading. Thus, it is generally desirable to use antistatic agents to reduce this static charge build-up.
Antistatic compositions must satisfy stringent requirements in order to be suitable for use in optical recording media. In addition to providing protection against the build-up of static charge, antistatic compositions must be transparent, abrasion resistant, and compositionally stable so that the compositions remain transparent for long periods of time. If the compositions become hazy, the amount of incident light that reaches the optical recording layer may be reduced, thus causing an increase in bit error rate, an increase in spare sector, or a loss of data. Another requirement concerns the viscosity of the antistatic composition. To obtain an antistatic coating of uniform thickness, the antistatic composition preferably must have low viscosity, e.g., 100 centipoise or less at 25.degree. C. The use of heat curable or hot thermoplastic compositions must also be avoided, since higher temperatures, e.g., temperatures greater than about 100.degree. C., can damage optical recording media and/or adversely affect media performance.
Generally, there are two approaches to using antistatic agents with the hard coat layer. One approach involves coating an antistatic agents directly onto a hard coat layer. This approach, however, generally does not provide long-lasting antistatic protection because coatings applied directly onto a hard coat do not adhere well and tend to be easily wiped away. Another approach involves pre-mixing antistatic agents into a hard coat composition before the hard coat layer is coated onto the media. With this approach, however, the conventional antistatic agents have been used at relatively high concentrations. High concentrations are required in order to provide acceptable antistatic protection, but the resulting hard coat layers tend to become hazy over time, show poor abrasion resistance, and delaminate from the media.
U.S. Pat. No. 5,176,943 (Woo) describes an antistatic hard coat composition comprising a nonionic perfluoro surfactant, an ionic perfluoro surfactant and a nonfluorinated copolymerizable radiation curable prepolymer, as well as optical recording disks comprising this antistatic hard coating.